(a) : Cyanobacteria are gram negative prokaryotes which are popularly known as blue-green algae. Although cyanobacteria are true prokaryotes, but their photosynthetic system closely resembles with that of eukaryotes because they have chlorophyll-a and photosystem II and they carry out oxygenic photosynthesis. Like the red algae, cyanobacteria use phycobiliproteins as accessory pigments. Photosynthetic pigments and electron transport chain components are located in thylakoid membranes lined with particles called phycobilisomes, which contain phycobilin pigments, particularly phycocyanin and transfer energy to photosystem II. They contain nitrogenous enzyme for nitrogen fixation. This enzyme becomes inactive in the presence of oxygen but the thick walled heterocyst provide suitable anaerobic environment for nitrogenous activity even in aerobic conditions.